The invention relates to administering apparatus and devices, including injection devices, and methods for administering a fluid product or substance. The main area of application or use is in human medicine. The extended area of application, however, also includes cosmetics and veterinary medicine. In particular, the invention relates to an administering apparatus for self-administering, i.e., a person administering a substance to him or her self, as is, for example, common in diabetes therapy or in administering growth hormones.
Injection apparatus in the form of so-called injection pens, in which the product is delivered from a product reservoir and administered with the aid of a piston, have become known and accepted for self-administering. In some examples, the piston is activated via a piston rod. The piston rod is in engagement with a drive device, in order—by activating the drive device—to move the piston rod and, together with it the piston, in an advancing direction and thus deliver product. Toothed rods are a common type of piston rods. Toothed rods comprise at least one row of teeth extending in the longitudinal direction of the toothed rod. The at least one row of teeth must permit the movement of the piston rod in the advancing direction and can form the engagement with the drive device. A piston rod is usually formed as a toothed rod when a movement of the piston rod counter to the advancing direction is not to be permitted. For this purpose, the row of teeth co-operates with a blocking means, which engages with the row of teeth and thus prevents a returning movement of the piston rod in any axial position which the piston rod can assume during undisrupted operation. Due to the function of the at least one row of teeth as a returning block, administering apparatus comprising piston rods formed as toothed rods are typically apparatus which are completely or partially disposed of once the reservoir has been completely emptied.
In order to ensure that a piston rod formed as a toothed rod is correctly installed, such a piston rod should only be installed in the apparatus by the apparatus manufacturer. Due to the interaction of the returning block, the engagement of the drive device or a dosing mechanism with the piston rod, errors can occur when installing such a piston rod, in particular if the piston rod is installed by a patient or end-user. If the piston rod is installed by the user, there is a danger of the piston rod not being installed correctly with respect to the parts of the administering apparatus to be engaged therewith.